In vitro studies have established that airway smooth muscle (ASM) cells exhibit phenotypic plasticity in response to inflammatory events mediated by transcriptional and post-transcriptional mechanisms that have not been fully elucidated. microRNA (miRNA)-mediated gene silencing has emerged as an regulator of gene expression and studies of miRNA function in ASM hold promise for the development of novel miRNA-based tools for prognosis and therapy. The long-term goal of our studies is to identify the miRNA-mediated gene- silencing mechanisms determining ASM cell phenotype in asthma. Our laboratory was the first to characterize miRNA expression in ASM following a pro-inflammatory stimulus and identified miR-25 as a target of the inflammatory response that regulates plasticity of ASM cells. This proposal will test the hypothesis that expression of miR-25 attenuates allergic asthma pathogenesis by altering ASM phenotype. In Specific Aim 1, we will determine whether miR-25 expression in a mouse model of allergic inflammation attenuates hyperreactivity, remodeling and alters ASM contractile force using a unique transgenic mouse model of smooth muscle-targeted miR-25 expression. These mice will be used to assess the effects of miR-25 on AHR, remodeling and ASM contractility following acute and chronic models of ovalbumin-sensitization and challenge. In Specific Aim 2, we will study the effect of smooth muscle-targeted miR-25 expression on increased ASM mass in vivo following acute and chronic ovalbumin-sensitization and challenge by assessing proliferative markers and mitogenic signaling pathways to address the molecular mechanisms underlying this miR-25 function. In Specific Aim 3, we will determine whether miR-25 affects the phenotype of asthmatic ASM cells using gain and loss of miR-25 function studies, as well as effects on identified targets of miR-25 mediated gene-silencing. This unique experimental approach, coupled with correlations in human diseased cells, will provide mechanistic data describing miR-25 function in the lung and be an essential step towards developing novel therapeutic strategies targeting miRNA in asthma.